News on the giant tube-snout sea turtle – Ocepechelon

The weird ones really are more interesting, aren’t they?
For awhile I’ve wanted to study this weird putative chelonid turtle to see where it nests in the large reptile tree (now at 580 taxa). It’s not a chelonid, as originally interpreted by Bardet et al. (2013). Ocepechelon bouyai OCP DEK/GE 516 (Fig. 1), the famous suction-feeding giant sea turtle, nests with the soft-shell turtles, Odontochelys (Late Triassic) and Trionyx (post-Cretaceous). They share a longer rostrum, and several other traits, that short-beaked chelonids just don’t have.

Figure 1. Ocepechelon bouyai OCP DEK/GE 516, the famous suction-feeding giant sea turtle.Note the large orange bones are supratemporals, not squamosals. The original premaxilla is an anterior nasal separated from the posterior portion by a new dorsal set of nares. The skull roof retains post parietals, The occiput includes tabulars, as seen in Trionyx. Click to enlarge.

Figure 1. Ocepechelon bouyai OCP DEK/GE 516, the famous suction-feeding giant sea turtle.Note the large orange bones are supratemporals, not squamosals. The original premaxilla is an anterior nasal separated from the posterior portion by a new dorsal set of nares. The skull roof retains post parietals, The occiput includes tabulars, as seen in Trionyx. Click to enlarge.

Apparently
Trionyx and Odontochelys were not tested in the original phylogenetic analysis of Bardet et al. 2013, which only used sea turtles in the inclusion set. That taxon exclusion has become a problem with a solution (Fig. 2).

Figure 2. The family tee of turtles with Ocepechelon nesting with soft-shell turtles.

Figure 2. The family tee of turtles with Ocepechelon nesting with soft-shell turtles. High bootstrap support figures here.Taxon exclusion prevented this nesting in Bardet et al. 2013.

The supplemental data
indicates Bardet et al. used an all-zero hypothetical outgroup and did not include Odontochelys or Trionyx. The supplemental data also includes a short movie of the turtle feeding on a fish, of which here is one frame (Fig. 3). Click here to view the very short movie on YouTube.

Ocepechelon feeding, one frame from the Bardet et al 2013 movie.

Ocepechelon feeding, one frame from the Bardet et al 2013 movie. Click to view on YouTube

A living turtle,
the long-necked mata-mata can be seen here suctioning it prey by expansion of the neck, with a wide open mouth. This is different, of course, from the pipette method used by Ocepechelon. The original purported ‘premaxilla’ is an anterior pair of nasals separated from the posterior pair by a newly opened dorsal set of nares. So rare this may be the only time this has happened for tetrapods.

Postcrania support
the supplemental material of Bardet et al. 2013 reports: “The Maastrichtian Phosphates of the Oulad Abdoun Basin have yielded several very large Chelonioid elements (OCP collection): dorsal shells with large pleural disc fontanelles, widely U-notched and and and indented nuchal, various star-shaped plastral elements with deeply indented edges, shoulder and pelvis (neither protostegid nor dermochelyid) elements, as well as humeri and femurs. All these postcranial elements from Morocco may correspond to Ocepechelon, the only skull morphotype known in the Oulad Abdoun with a corresponding large size. As the skull, none of these postcranial elements can be surely referred to Protostegidae or Dermochelyidae and they could partly belong to Ocepechelon.”

So, the skull of Ocepechelon is not chelonioid, according to the large reptile tree and the disassociated post-crania is not chelonioid, according to the authors. Does anyone else want to look into the possibility that we have a giant soft-shell turtle here? Just add Ocepechelon to a large gamut turtle matrix. Publish the post-cranial bits and pieces. And fix those squamosal/supratemporal issues while you’re at it.

References
Bardet N, Jalil N-E, de Lapparent de Broin F, Germain D, Lambert O, et al. 2013. A Giant Chelonioid Turtle from the Late Cretaceous of Morocco with a Suction Feeding Apparatus Unique among Tetrapods. PLoS ONE 8(7): e63586. doi:10.1371/journal.pone.0063586
Li C, Wu X-C, Rieppel O, Wang L-T and Zhao L-J 2008.
An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.
Gaffney ES 1975. A phylogeny and classification of higher categories of turtles. Bulletin of the American Museum of Natural History 155:387-436.
Meylan PA 1987. The phylogenetic relationships of the soft-shelled turtles (family Trionychidae). Bulletin of the American Museum of Natural History 186:1-101.

wiki/Trionyx
wiki/Odontochelys
wiki/Ocepechelon

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